4.4.3.2.1.8 DMCDB – Demodulator Carrier Detect for Path B

The DMCDB register contains the settings for the carrier detection of path B. The number of checked samples (DMCDB.DMCTB) and the maximum number of allowed errors (DMCDB.DMCLB) within this time window can be configured. This register must only be modified if the block receiving the settings is disabled (RDPR.PRPTB = 1). Modifying the settings during operation may lead to unstable operation.

Name:	DMCDB
Offset:	0x0A3
Reset:	0x00


Bit	7	6	5	4	3	2	1	0
	DMCTB[2:0]			DMCLB[4:0]
Access	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Reset	0	0	0	0	0	0	0	0

Bits 7:5 – DMCTB[2:0] Demodulator Carrier Detect Time for Path B

This setting defines the time window for carrier detection as a number of checked samples. A counter accumulates the carrier errors for the number of samples defined in this register. The counter is reset after this period for the next check cycle. Selecting DMCTB = 0 turns the carrier check on path B off. Microchip recommends selecting a DMCTB value covering 2 symbols (one Manchester bit).

Calculation of the number of samples to span over 2 symbols:

S A M P L E S_2 S Y N B = 2 \times \frac{C L K_B B}{S y m b o l R a t e P a t h B} (32)

Equation parameters:
- CLK_BB is the baseband clock frequency in Hz. See equation (13) in Bandwidth Scaling. See Bandwidth Scaling in the Channel Filter from Related Links.

Choose a number of samples greater than or equal to SAMPLES_2SYNB from the following table. The corresponding carrier detect time DMCTB[2:0] value is the preferred setting.

S A M P L E S_C T B \geq S A M P L E S_2 S Y N B

Table 4-19. Number of Samples for Carrier Detect Time versus DMCTB Setting
SAMPLES_CTB	DMCTB
Carrier Detect OFF	0
16	1
32	2
64	3
128	4
256	5
512	6
1024	7

Bits 4:0 – DMCLB[4:0] Demodulator Carrier Detect Limit for Path B

Sets the number of allowed carrier errors within the time window defined by DMCTB. The recommended value for this limit is 20% of the samples within the carrier detect time window (see SAMPLES_CTB value above). Smaller values, for example, 10%, lead to more rigorous checking, resulting in more rapid error detection. More rigorous checking may cause reduced sensitivity.

Calculation procedure example:

Select the number of allowed errors within the carrier detect time window (20%).
$A l l o w e d C a r r i e r C h e c k E r r o r s B = \frac{S A M P L E S_C T B}{5} (34)$

Equation parameters:
- SAMPLES_CTB: Number of samples for the carrier check (see the preceding table).

Calculate the register setting
$D M C L B = r o u n d (\frac{A l l o w e d C a r r i e r C h e c k E r r o r s B}{4}) (35)$